Experiment admin: visually signify hierarchy in Phases/Blocks

[BeritJanssen]

Right now, there is slight indentation in the Phases/Blocks form in the Experiment admin, which is too subtle to indicate the hierarchy in these models.

Solutions suggested:

• arrows
• colours / opacity
• more indentation
• lines
• icons

Example with arrows and indentation:

[drikusroor]

I've put this one on hold for now as I think it's probably better if I wait for #1321 to be finished to avoid merge conflicts, but also to potentially re-use the tabs for the individual blocks.